There are already enough leaks of what will be appearing at E3 that I think we can start talking about it already.
Black Ops 3
505/Overkill's Walking Dead game
Star Wars Battlefront
Nintendo Star Fox
Those are all confirmed.
Rumors are Gears Of War remasters (test footage leaked yesterday), a Fallout 4, Zelda Wii U, and possibly a new Elder Scrolls game.
Nintendo is once again not doing a traditional press conference and instead will do another digital event that can be streamed online. I think not doing a showcase for the 2nd year in a row is kind of a deathblow to the company even though they are expecting to announce their new mobile gaming device.